This invention relates to electric propulsion systems and more particularly to such systems for use in submarines.
Electric submarine drive systems must operate at high efficiency while maintaining smooth acceleration and including means for braking. These requirements are met by a system which alternatively connects a plurality of electric motors in a series or parallel configuration. Vehicle propulsion systems utilizing series/parallel motor circuits are well known. Examples of such systems can be found in U.S. Pat. No. 3,369,162, issued Feb. 13, 1968, to Yasunosuke Torii; U.S. Pat. No. 3,764,867, issued Oct. 9, 1973 to Smith; and U.S. Pat. No. 3,800,197, issued Mar. 26, 1974 to Mehta.
A particular vehicle propulsion system used on mine shuttle cars utilizes a series/parallel motor arrangement and includes a branch circuit comprising the series connection of two series wound d.c. motors connected between two power supply conductors. A diode is provided between the motors, one contactor is connected between the anode of the diode and one power conductor while a second contactor is connected between the cathode of the diode and the other power conductor. Closure of these contactors switches the motors from series to parallel operation. This system provides some control over the motor acceleration but is not designed for accurate speed control.
U.S. Pat. No. 1,342,797, issued June 8, 1920 to Conti discloses an early marine propulsion system which utilizes a series/parallel motor arrangement. In that system, two shunt wound d.c. motors are connected in series between two power supply conductors. A first contactor is connected in series between the motors and a second contactor is connected between one side of the first contactor and one power supply conductor while a third contactor is connected between the other side of the first contactor and the other power supply conductor. For series operation, the first contactor is closed while the second and third contactors are open. To switch to parallel operation, the first contactor must be opened and a certain period of time must pass to ensure that current through the first contactor has ceased before the second and third contactor can be closed. During this time interval, the motors slow down and may have to be reaccelerated to their previous series operation speed. In addition, some type of mechanical interlock may be used to ensure proper contactor sequencing. This increases system complexity and decreases reliability.
The use of direct current shunt wound electric motors in a series/parallel arrangement provides for a wider range of controllable speeds than a single motor propulsion circuit and produces a more desirable family of speed torque curves for propeller-type loads than a series/parallel arrangement of series wound direct current motors. For electric battery powered submarine applications, it is desirable that the propulsion system include regenerative braking. In addition, it must have provisions for low speed maneuvering and include means for reducing electromagnetic interference.
An electric motor marine propulsion system constructed in accordance with the present invention includes at least two direct current shunt wound electric motors and means for alternatively connecting the motors in series/parallel configurations. A diode is connected between the motors to provide for a smooth transition from series to parallel operation. Means for shorting this diode is included to allow regenerative braking. Additional motor speed control is provided by adjusting field current in the motor shunt windings and/or regulating voltage or current delivered to the circuit.